Water & Nutrient Management:
Soil Moisture-Based Irrigation Systems
Volumetric Sensors for Soil Moisture:
Frequency Domain Reflectometry (FDR)
Working Principle
The electrical capacitance of a capacitor that uses the soil as a dielectric depends on the soil water content. When connecting this capacitor (made of metal plates or rods imbedded in the soil) together with an oscillator to form an electrical circuit, changes in soil moisture can be detected by changes in the circuit operating frequency. This is the basis of the Frequency Domain (FD) technique used in Capacitance and Frequency Domain Reflectometry (FDR) sensors. In Capacitance sensors the dielectric permittivity of a medium is determined by measuring the charge time of a capacitor made with that medium. In FDR the oscillator frequency is swept under control within a certain frequency range to find the resonant frequency (at which the amplitude is greatest), which is a measure of water content in the soil.
Description
Probes usually consist of two or more electrodes (i.e., plates, rods, or metal rings around a cylinder) that are inserted into the soil. On the ring configuration the probe is introduced into a access tube installed in the field. Thus, when an electrical field is applied, the soil around the electrodes (or around the tube) forms the dielectric of the capacitor that completes the oscillating circuit. The use of an access tube allows for multiple sensors to take measurements at different depths.
Advantages
- Accurate after soil-specific calibration (±0.01 ft3ft-3)
- Can read in high salinity levels, where TDR fails
- Better resolution than TDR (avoids the noise that is implied in the waveform analysis performed by TDRs)
- Can be connected to conventional loggers (DC output signal)
- Flexibility in probe design (more than TDR)
- Some devices are relatively inexpensive compared to TDR due to use of low frequency standard circuitry
Drawbacks
- The sensing sphere of influence is relatively small (about 1.6 in.)
- For reliable measurements, it is extremely critical to have good contact between the sensor (or tube) and soil
- Careful installation is necessary to avoid air gaps
- Tends to have larger sensitivity to temperature, bulk density, clay content and air gaps than TDR
- Needs soil-specific calibration
- http://vfd.ifas.ufl.edu/gainesville/irrigation/frequency_domain_reflectometry_probe.shtml
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